Statorade

Statorade

Statorade is a line of low viscosity magnetically suspended motor cooling fluids that is being jointly developed by Grin Tech, a pioneer in ebike R&D, and Ferrotech, the inventor and undisputed leader of Ferrofluid science since the 1960’s.

How it Works

The idea behind Statorade is brilliant and simple. A direct drive hub motor generates heat internally at the stator, but this heat has no easy path to escape since it is surrounded by trapped air. It is possible to cool the motor with vent holes and active airflow from outside, but this exposes the motor to the elements. Alternately, filling the motor with oil will cool the stator windings quite well, but with significant increase in motor resistance from fluid drag and an enormous sealing challenge to keep the oil from leaking

With Statorade, only a tiny amount of fluid is needed to achieve the same effect. Nanomagnetic particles suspended in a low-viscosity oil concentrate this liquid in the magnetic air gap of the motor. That is exactly where it needs to be to transfer heat through fluid convection between the motor stator to the motor shell, where the heat can then be easily shed to the environment via passing airflow.

Statorade Ferrofluid Heat Flow Diagram

The magnets on the rotor trap the fluid in the gap so that it can’t splash around and leak out through the ball bearings or cable wiring as happens with normal attempts at oil cooling. And it turns out that only a tiny amount of Statorade fluid is needed to achieve heat transfer, resulting in negligible effect on the rolling drag of the motor.

How Well Does It Work?

Effects of Statorade Ferrofluid on TDCM IGH Hub Motor

The short answer is very well, on average roughly doubling the thermal conductivity from the copper windings to the outside environment. That is similar to the cooling performance improvement achieved by drilling very large vent holes on both side cover plates, but without any of the exposure consequences. At low speed Statorade does a slightly better job, while at high speeds the active air flow has an edge. 

In the graph on the left, you can see the effects of adding Statorade to the TDCM 5 Speed IGH hub motor in 1mL increments. By the time we get to 6mL of Statorade added to the motor, the thermal conductivity from the copper to the motor shell has increased from 1.8 to 4.3 Watts/Deg, and the steady state motor core temperature has reduced from nearly 90 °C to less than 60 °C.  

 

What does this mean in practice? Well, if you can double the amount of heat that a motor can dissipate, it means (due to the I2R relationship) you can run it at roughly 40% higher sustained torque and power without overheating. 40%! In practice, what this means is that bike rides that used to push your hub motor to 100+°C will now only result in ~70°C temperatures. And when the motor does get hot from a short intense power burst, then it be that much faster at cooling down and being ready for the next thrust of high power.

Of course, with a given motor controller and battery setup the motor isn’t going to feel any different once you add Statorade, off the line torque and everything will be exactly the same. It’s only when you are monitoring the motor temperatures that you truly see and appreciate the effect, and can expand your usage window accordingly.

We’ve created a very powerful ebike simulation tool that takes all of the firsthand data from our wind tunnel experiments and modeled a full hub motor and ebike environment around it. You can then directly compare any given trip simulation for a motor with and without Statorade addition and see for yourself how the core temperatures are reduced.

How Much to Use?

Statorade in MXUS Hub Motor, Fill Level vs Drag

One of the most fascinating aspects of Statorade is just how little is required to achieve these thermal benefits. With conventional oil cooling of a hub motor people would typically add several hundred mL of ATF oil or similar in order to get good thermal linkage which results in measurable extra drag from fluid friction. With Statorade, we start to see measurable improvements in heat dissipation with just 2-3 mL, and on most hub motors the full effect is reached with somewhere between 5-8 mL at modest RPMs, and 8-12mL for fast RPMs. If more Statorade is added beyond this, then there is almost no change in the heat flow but you will start getting increasing viscous fluid drag. In the graph below, you can see the effects of adding Statorade 1mL at a time on a large 40mm wide MXUS direct drive hub motor. 

If you aren't sure of the ideal fill amount for your hub, then we recommend measuring the no-load current draw of your motor at full throttle, then adding the Statorade in small increments at a time until you start to see a measurable increase in the unloaded current. This is the point where additional Statorade will mostly increase your rolling drag without adding much thermal benefit.  

 

Hubsinks

While Statorade does an excellent job of forming a thermal bridge between the motor core and the spinning shell, it doesn't do anything to assist moving heat from the shell itself to ambient air. In general the passing airflow from riding the bike will do a very good job of this. But for maximum cooling effect the addition of fins to the motor shell will provide further improvement, increasing the total heat flow by 15-20% over just Statorade alone. Matt from Strike3productions has created an innovative Hubsink product that is designed exactly for this purpose. The segmented fins can be inserted between the spokes of a laced wheel and bolt together, providing a finned surface to facilitate heat shedding from the motor shell.

HubsinksHubsinks Installed

The graphs below do a great job of showing just how Hubsinks work based on our testing of a Cromotor. On the left you see the core to shell conduction. The addition of 12mL Statorade to the core causes a huge increase in core to shell heat transfer, while hubsinks on top of this don't do anything. In the middle graph we are looking at the heat conductivity from the motor shell to ambient air. The addition of Statorade has virtually no effect, while the Hubsinks result in a 50-60% improvement over the natural conduction of a bare motor shell. Finally in the third graph we see the total effects on the net heat flow out of the motor, with 12mL Statorade improving things by ~80% and Hubsinks putting another 20% improvement over this.

Effect of Hubsinks and Statorade on Cromotor

Test Results

Link to Statorade Study in BionX Motors

We've characterized the effects of Statorade for improved motor cooling in over a dozen different hub motor lines, from tiny skateboard motors, to small Brompton kit motors like the SAW20, to very large hub monstrosities like the Cromotor. Many of those test graphs are shown below. We also did a detailed study of the improvements that could be expected from Statorade use in the BionX motor series.

This was an application of strong interest to us since the improved motor cooling would also help with controller performance, as the motor controller is built inside these hubs. The paper also demonstrates how to use our simulator tools to model the expected real-world improvements in motor torque, power, and hill climbing speed when the hub is in thermal rollback.

 

 

Frequently Asked Questions:

Will this work in geared hub motors or mid-drive motor systems?
No, not really. Geared hub motors do not have their rotor shell exposed to ambient air, so increasing the heat conductivity from the motor stator to the rotor does not help get the heat to ambient air outside. Similarly, most mid-drive motors (like the Bafang BBSXX) are inrunners, where the magnets and rotor are on the inside and the heat generating stator, and conducting heat to them would serve no purpose. The benefits of Statorade are really only present for outrunner motors (magnets on the outside) where the rotor is exposed to air flow.  
Is this compatible with all direct drive motors
In principle yes, but many motors will not have adequate sealing on the side cover plates which can allow some seepage and leakage of the fluid when the motor is running fast. And most motors won't have an injection port for adding Statorade, you will need to either drill a hole yourself or add it when the side cover is off and then seal it up. The Direct Drive motors offered at Grin are almost all pre-sealed and have statorade fill ports either on the disk holes or elsewhere on the side plate or axle. 
What about electric skateboard hub motors?
Skateboard hub wheels are also direct drive motors and so can benefit from Statorade in order to help bring heat away from the stator core. However, since the rotor of a skateboard motor is thermally blanketed by a the urethane rubber wheel, the resulting gains are not as significant as we see in ebike hub motors where the rotor metal is directly exposed to air flow. Typically the net improvement is about 20 percent. As well, the higher RPM or ebike motors means that the fluid is more likely to hit the centripetal force limit where it has less of an effect. At speeds of 20kph or less it can be helpful, while over 30kph the motor RPM is too high for the fluid to stay in the gap. The graphs below show test results for two popular skateboard drives.
What is the maximum RPM where this will work?
At higher speeds, a larger quantity of Statorade is required in order to make a successful bridge between the stator core and rotor magnets as centripetal forces tend to flatten the fluid and flight it out of the air gap. Our experiments indicates that it still works well with centripetal forces up to about 10-20G, while beyond 40G the effect is diminished unless extra Statorade fill is added. The table below shows the relationship between G forces and RPM for a number of motor diameters.
Relationship Between G Force and Motor RPM 
G Force 30 mm 50 mm 75mm 100mm 200mm 300mm
5G 390 rpm 300 rpm 240 rpm 210 rpm 150 rpm 120 rpm
10G 550 rpm 420 rpm 350 rpm 300 rpm 210 rpm 170 rpm
20G 770 rpm 600 rpm 490 rpm 420 rpm 300 rpm 240 rpm
40G 1090 rpm 850 rpm 690 rpm 600 rpm 420 rpm 350 rpm
100G 1730 rpm 1340 rpm 1090 rpm 950 rpm 670 rpm 550 rpm
 
How long does it last? Do I need to refill or top up on a regular basis?
The short answer is 1) a long time and 2) no, assuming that the motor side cover plates are well sealed against leakage. Statorade has been formulated with both a stable synthetic base and surfactants to maintain its properties even under prolonged exposure to elevated temperatures and agitation. From Dec. 2017 to March 2018th we conducted a accelerated life tests with Statorade in a direct drive motor spinning at 400 rpm with a motor core running at 120 degrees. When filled with just 6mL of fluid (not quite sufficient for full effect at this speed) we noticed a performance reduction over time equivalent to loosing about 0.3 mL of fluid every 10,000 km. On a later test when we started off with 9mL, we barely noticed any reduction in performance in the first 50,000 km of travel. Longevity Tests of Statorade in MXUS Motor

Be aware that inexpensive ferrofluids sold on ebay and the likes for demonstration / scientific purposes do not hold up at all in this motor application. Those are formulated for low cost and maximum visual effect, rather than long term stability at high temperatures and agitation levels.

Will this magnetic properties affect the motor speed or torque by allowing more flux through the air gap?
No. Although there have been some studies in motors with large air gaps where it was shown that ferrofluids could reduce the magnetic permeability and increase flux, this is not the intention of Statorade. Firstly, Statorade only occupies a small portion of the air gab volume, and secondly, it has been optimized for low viscosity and thermal transfer rather than conducting flux. Even if you ignore our quantity recommendations and completely fill the entire air gap with Statorade, you will not observe a measurable change in the motor's magnetic characteristics.

Background

This discovery and research into ferrofluids for motor cooling resulted from our in depth thermal studies of hub motors, much of which is documented on the endless-sphere topic "Definitive Tests on the Heating and Cooling of Hub Motors" . We started this thread in early 2013 in order to characterize hub motor thermal behaviors, both to make a more accurate simulator model and to quantify the effects of air vent and oil cooling mods both of which were popular with hot-rodders at the time.

The thread is fairly lengthy but a good read. Some of the more interesting posts are:

Accounting for Motor Core Hysteresis Loss
Test Results in Still Air
Experiments with Internal Blades
Measuring Motor Core and Shell Heat Capacities
Concept for Internal Heat Transfer Fins
Building of Wind Tunnel
Infrared Camera Images
Observations of Drag Torque
Experiments with Potted Motor Core
Combinations of Vents + Statorade

Videos

Here is a video showing one method of adding Statorade to a hub motor without opening the motor first

Here is a video showing the effects of riding with and without Statorade on a small direct drive hub motor.

Deployment and Availability

Our goal was to have all direct drive motors supplied by Grin include Statorade by the end of 2016 and we have succeeded on this front. We started off with it in the TDCM internal geared hubs at the start of the year and shortly after introduced it in the 9C+ motor series. In late summer 2016 all of our new Crystalyte motors were Statorade compatible as well, with silicone sealant on the motor side plates and a through-hole inside one of the disk bolt holes for injecting the Statorade.And our more recent MXUS and Grin All-Axle hub motors are both sealed with Statorade injection ports. 

We also offer stand alone 10mL syringes of Statorade to anyone who wants to experiment with adding it to their existing direct drive hub motor, as well as 50mL and 100mL bottles for those using it in larger quantities.  The 10mL syringes are also available directly via Amazon for US customers. We have an OEM program for any motor manufacturers who want to purchase and consume it in volume.

Where to Order